U.S. patent number 5,165,080 [Application Number 07/654,000] was granted by the patent office on 1992-11-17 for optical distributor.
This patent grant is currently assigned to British Telecommunications public limited company. Invention is credited to Peter Healey.
United States Patent |
5,165,080 |
Healey |
November 17, 1992 |
Optical distributor
Abstract
An optical distributor comprises an optical input (Il), and
2.sup.n optical outputs (01-08) , where n is an integer greater
than 1, one or more plane mirror surfaces (8) and transflective
means (2, 4, 6). The plane mirror surface or surfaces (8) and the
transflective means (2, 4, 6) are arranged to provide one optical
path between the input (Il) and each of the outputs (01-08), each
path making n transitions only at the transflective means. In a
preferred embodiment the transflective means comprise n separate
beam splitter stages, the effective lengths of the n separate
splitter stages (2, 4, 6) being substantially in the ratio 1.2
2.sup.n. The beam splitters may be formed by transflective coatings
on glass substrates.
Inventors: |
Healey; Peter (Ipswich,
GB2) |
Assignee: |
British Telecommunications public
limited company (London, GB2)
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Family
ID: |
26292720 |
Appl.
No.: |
07/654,000 |
Filed: |
February 13, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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358358 |
May 11, 1989 |
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Foreign Application Priority Data
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Sep 11, 1987 [GB] |
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8721472 |
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Current U.S.
Class: |
359/636 |
Current CPC
Class: |
G02B
6/2817 (20130101); G02F 1/31 (20130101) |
Current International
Class: |
G02F
1/31 (20060101); G02F 1/29 (20060101); G02B
6/28 (20060101); H04Q 3/52 (20060101); G02B
027/14 () |
Field of
Search: |
;350/171,173,169
;359/636 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2454996 |
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May 1976 |
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DE |
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2341873 |
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Oct 1976 |
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FR |
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2187861A |
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Sep 1987 |
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GB |
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Other References
Japanese Patent Abstracts, vol. 7, No. 73 Mar. 25, 1983. .
Journal of Lightwave Technology, Vo LT-3, No. 2, Apr. 1985
4.times.4 Optical Gate Matrix Switch, Himeno et al..
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Primary Examiner: Henry; Jon W.
Attorney, Agent or Firm: Nixon & Vanderhye
Parent Case Text
This is a continuation of application Ser. No. 07/358,358, filed as
PCT/GB88/00746, Sep. 9, 1988, now abandoned.
Claims
I claim:
1. An optical distributor having:
one optical input device and 2.sup.n optical output devices, where
n is an integer greater than 1,
said distributor being adapted to split an optical signal received
at said input device into 2.sup.n optical signals distributed one
to each of said optical output devices, whereby said distributor
provides one complete optical beam path from said input device to
each of said output devices;
wherein said distributor also comprises:
one or more plane mirror surfaces, and two or more transflective
surfaces;
said mirror and transflective surfaces being arranged such
that:
the intersections of said complete beam paths and said
transflective surfaces define 2.sup.n -1 transflective means,
each of said transflective means splits an incident beam path into
a reflected beam path and a transmitted beam path,
every complete beam path makes exactly n transitions of said
transflective means, and
at least one of said transflective surfaces contains at least two
of said transflective means.
2. An optical distributor having:
one optical input device and 2.sup.n optical output devices where n
is an integer greater than 2,
said distributor being adapted to split an optical signal received
at said input device into 2.sup.n optical signals distributed one
to each of said optical output devices, whereby said distributor
provides one complete optical beam path from said input device to
each of said output devices;
wherein said distributor also comprises:
one or more plane mirror surfaces, and n transflective
surfaces;
said mirror and transflective surfaces being arranged such
that:
the intersections of said complete beam paths and said
transflective surfaces define 2.sup.n -1 transflective means,
each of said transflective means splits an incident beam path into
a reflected beam path and a transmitted beam path,
every complete beam path makes exactly n transistions of said
transflective means, and
one of said transflective surfaces contains 2.sup.n-1 of said
transflective means.
3. An optical distributor having:
one optical input device and 2.sup.n optical output devices where n
is an integer greater than 2,
said distributor being adapted to split an optical signal received
at said input device into 2.sup.n optical signals distributed one
to each of said optical output devices, whereby said distributor
provides one complete optical beam path from said input device to
each of said output devices;
wherein said distributor also comprises:
one or more plane mirror surfaces, and n transflective
surfaces;
said mirror and transflective surfaces being arranged such
that:
the intersections of said complete beam paths and said
transflective surfaces define 2.sup.n-1 transflective means, namely
one first transflective surface, one last transflective surface and
(n-2) intermediate transflective surfaces,
each of said transflective means splits an incident beam path into
a reflected beam path and a transmitted beam path,
every complete beam path makes exactly n transitions of said
transflective means,
said first transflective surface contains one transflective
means,
said last transflective surface contains (2.sup.n-1) transflective
means, each having its reflective beam path and its transmitted
beam path connected to different ones of said output devices such
that each output device is connected to one of said transflective
means,
for the or each integral value of m in the range m=1 to m=n-2
inclusive; there is an intermediate transflective surface which
contains 2.sup.m of the transflective means.
4. An optical distributor according to claim 3, which has one
mirror surface, and all of said transflective surfaces are parallel
to said mirror surface.
5. An optical distributor according to claim 4, wherein the
transmitted beam from each transflective means in said first
transflective surface and the or each intermediate transflective
surface goes directly to another transflective means and each
reflected beam from said transflective means goes to another
transflective means by way of reflection from said mirror.
6. An optical distributor according to claim 3, wherein all of said
transflective surfaces are coplanar, whereby all of said
transflective means are located in a single transflective plane,
and each of said transflective surfaces is located between and
parallel to two mirror surfaces.
7. An optical distributor having:
2.sup.n optical input devices and 2.sup.n optical output devices,
where n is an integer greater than 1,
said distributor being adapted to split an optical signal received
at any one of said input devices into 2.sup.n optical signals
distributed one to each of said optical output devices, whereby
said distributor provides one complete optical beam path from each
of said input devices to each of said output devices;
wherein said distributor also comprises:
one or more plane mirror surfaces, and two or more transflective
surfaces;
said mirror and transflective surfaces being arranged such
that:
the intersections of said complete beam paths and said
transflective surfaces define n2.sup.n-1 transflective means,
each of said transflective means has two primary paths and two
secondary paths, such that an optical signal received by either one
of said primary paths is provided as a reflected signal in one of
said secondary paths, and as a transmitted signal in the other of
said secondary paths,
every complete beam path makes exactly n transitions of said
transflective means, and
at least one of said transflective surfaces contains at least 2 of
said transflective means.
8. An optical distributor having:
2.sup.n optical input devices and 2.sup.n optical output devices,
where n is 154 integer greater than 1,
said distributor being adapted to split an optical signal received
at any one of said input devices into 2.sup.n optical signals
distributed one to each of said optical output devices, whereby
said distributor provides one complete optical beam path from each
of said input devices to each of said output devices;
wherein said distributor also comprises:
one or more plane mirror surfaces, and two or more transflective
surfaces;
said mirror and transflective surfaces being arranged such
that:
the intersections of said complete beam paths and said
transflective surfaces define n2.sup.n-1 transflective means,
each of said transflective means has two primary paths and two
secondary paths, such that an optical signal received by either one
of said primary paths is provided as a reflected signal in one of
said secondary paths, and as a transmitted signal in the other of
said secondary paths,
every complete beam path makes exactly n transitions of said
transflective means, and
at least one of said transflective surfaces contains 2.sup.n-1 of
said transflective means.
9. An optical distributor having:
2.sup.n optical input devices and 2.sup.n optical output devices
where n is an integer greater than 2,
said distributor being adapted to split an optical signal received
at any one of said input devices into 2.sup.n optical signals
distributed one to each of said optical output devices, whereby
said distributor provides one complete optical beam path from each
of said input devices to each of said output devices;
wherein said distributor also comprises:
one or more plane mirror surfaces, and n transflective
surfaces;
said mirror and transflective surfaces being arranged such
that:
the intersections of said complete beam paths and said
transflective surfaces define n2.sup.n-1 transflective means,
each of said transflective means has two primary paths and two
secondary paths, such that an optical signal received by either one
of said primary paths is provided as a reflected signal in one of
said secondary paths, and as a transmitted signal in the other of
said secondary paths,
every complete beam path makes exactly n transitions of said
transflective means, and
each of said transflective surfaces contains 2.sup.n-1 of said
transflective means.
10. An optical distributor having:
one optical input device, and 2.sup.n optical output devices, where
n is an integer greater than 2, said distributor being adapted to
split an optical signal received at said input device into 2.sup.n
optical signals and to provide one of said signals to each of said
output devices whereby said distributor provides one complete
optical path from said input device to each of said optical
devices,
wherein said distributor also comprises:
one or more plane mirror surfaces, and
a sequence of n transflective stages said sequence including one
first transflective stage; (n-2) intermediate transflective stages,
and, one last transflective stage
wherein:
said first transflective stage includes one transflective means
whereat the one beam path defined by the input device is split into
a reflected beam path and transmitted beam path;
the or each of said intermediate transflective states has a
predecessor transflective stage being either the first
transflective stage or another of said intermediate transflective
stages and a successor transflective stage being either the last
transflective stage or another intermediate transflective stage
different from said predecessor transflective stage and each of
said intermediate transflective stages contains twice as many
transflective means as its predecessor transflective stage; and
said last transflective stage has the or one of said intermediate
transflective stages as its predecessor transflective stage and it
includes 2.sup.n-1 transflective means each of which splits an
incident beam path into a reflected beam path directed to one of
said output devices and a transmitted beam path directed to a
different one of said output devices whereby each output device is
located on one of said beam paths;
wherein:
each transflective means contained in the or each intermediate
transflective stage has an incident beam path directed from a
transflective means in the predecessor transflective stage said
incident beam path being split into a reflected beam path forming
the incident beam path to one of the transflective means in the
successors transflective stage and a transmitted beam path forming
the incident beam path of a different one of the transflective
means in said successor transflective stage,
and wherein:
all of the transflective means contained in at least one of said
intermediate and last transflective stages are located on a single
transflective surface.
11. An optical distributor according to claim 10, which comprises
one transflective surface corresponding to each one of said
transflective stages wherein each of said transflective surfaces
contains the or all of the transflective means in the corresponding
transflective stage.
12. An optical distributor according to claim 11, which has one
mirror surface, and all of said transflective surfaces are parallel
to said mirror surface.
13. An optical distributor according to claim 11, wherein all of
said transflective surfaces are coplanar, whereby all of said
transflective means are located in a single transflective plane,
and each of said transflective surfaces is located between and
parallel to two mirror surfaces.
14. An optical distributor having:
2.sup.n optical input devices, and 2.sup.n optical output devices,
where n is an integer greater than 2, said distributor being
adapted to split an optical signal received at any one of said
input devices into 2.sup.n optical signals and to provide one of
said signals to each of said output devices whereby said
distributor provides one complete optical path from each said input
device to each of said output devices,
wherein said distributor also comprises one or more plane mirror
surfaces, and
a sequence of n transflective stages each of which contains
2.sup.n-1 transflective means and said sequence includes one first
transflective stage, (n-2) intermediate transflective stages, and,
one last transflective stage;
wherein:
each of said transflective means has two primary paths and two
secondary paths, such that an optical signal received by either one
of said primary paths is provided as a reflected signal in one of
said secondary paths, and as a transmitted signal in the other of
said secondary paths,
the one beam path defined by each of the input devices is received
as one of the primary beam paths of one of the transflective means
in said first transflective stage;
the or each of said intermediate transflective stages has a
predecessor transflective stage being either the first
transflective stage or another of said intermediate transflective
stages and a successor transflective stage being either the last
transflective stage or another intermediate transflective stage
different from said predecessor transflective stage;
said last transflective stage has the or one of said intermediate
transflective stages as its predecessor transflective stage and
each secondary beam path of each transflective means contained in
said last transflective stage is directed to one of said output
devices whereby each output device is located on one of said beam
secondary paths;
wherein:
each transflective means contained in the or each intermediate
transflective stage has its primary beam paths directed from
different transflective means in the predecessor transflective
stage and its secondary beam paths directed to different
transflective means in the successor transflective stage;
and wherein:
all of the transflective means contained in at least one of said
first, intermediate and last transflective stages are located on a
single transflective surface.
15. An optical distributor according to claim 14, which comprises
one transflective surface corresponding to each one of said
transflective stages wherein each of said transflective surfaces
contains the or all of the transflective means in the corresponding
transflective stage.
16. An optical distributor according to claim 15, wherein all of
said transflective surfaces and all of said mirror surfaces are
parallel.
17. An optical distributor according to claim 11, wherein all of
said transflective surfaces are coplanar, whereby all of said
transflective means are located in a single transflective plane,
and each of said transflective surfaces is located between and
parallel to at least two mirror surfaces.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to devices for distributing optical
signals. Such devices are needed in fields such as optical
communications and signal processing. They may be required to split
only one input signal amongst 2.sup.n output devices or
alternatively may interconnect 2.sup.n input devices with 2.sup.n
output devices, n being an integer greater than 1. Known devices
used for this purpose have typically been formed from arrays of
4-port optical couplers interconnected by links of optical fibres
in a star network configuration. These devices require large
numbers of separate components, as many as 12 different couplers
being required in the case of an 8 port star network. The physical
complexity of such systems makes them expensive to manufature and
difficult to install in-line in optical systems. Moreover,
insertion losses associated with each individual coupler have a
cumulative effect which significantly degraded the performance of
the network as a whole.
FR-A-2,341,873 and DE-A-2454,996 disclose a method of forming a 1
to 2.sup.n optical distributor which uses n successive stages of
optical beam splitters, each beam splitter stage having one or more
individual transflective beam splitters to split an optical path
into two further paths, and mirrors to realign the optical paths as
necessary.
Such devices are capable of splitting an optical signal equally
among many outputs and of distributing intact all the components of
a signal which is multidimensional in the optical regime, having
spatial, colour, time and polarisation components but it uses an
arrangement which requires 2.sup.(k-1) separate beam splitters
components for the k.sup.th stage and associated mirrors which
means the number of beam splitters elements increases rapidly with
increasing n.
According to a first aspect the present invention an optical
distributor has at least one optical input, 2.sup.n optical
outputs, where n is an integer greater than 1, one or more plane
mirror surfaces and at least one transflective means, the plane
mirror surface or surfaces and the transflective means being
arranged to provide one optical path between the input and each of
the outputs, each path making n transitions only at the
transflective means, at least two of the transflective means being
portions of a single transflective surface.
A device in accordance with the present invention is more compact,
and may be formed using a smaller and sometimes the minimum number
of different components than with previously known arrangements and
can therefore be simpler and cheaper to manufacture and install.
The two transflective means may be part of the same or different
beam splitter stages.
The present invention is applicable to distributors having a single
input to provide a 1 to 2.sup.n distributor, which is functionally
equivalent to known such distributors as previously described, as
well as to distributors where the above optical input is one of
2.sup.n optical inputs, for example star couplers.
The transflective means may, for example, be formed by a
transflective coating on a glass substrate or by a cube beam
splitter. They have the property that light incident upon the
transflective means is divided into two beams of equal power, a
reflected beam and a transmitted beam. The transflective means are
substantially insensitive to the presence of different polarisation
components in the incident light.
The or each plane mirror surface may be formed by a conventional
metal or multilayer dielectric coating on a glass substrate, by a
totally reflecting prism or by any other surface arranged to
reflect substantially all of the power of any incident beam.
A distributor according to the present invention having one input
and 2.sup.n outputs can be arranged to n beam splitter stages each
of which includes a single separate transflective surface and a
first plane mirror surface arranged such that the or each beam
incident on each beam splitter stage is split into a reflected beam
and a transmitted beam, one of the beams passing directly to any
subsequent beam splitter stage, the other of the two beams being
reflected at the first plane mirror surface before passing to any
subsequent beam splitter stage.
Preferably the transflective surfaces are arranged to lie at
substantially 45.degree. to the incident beams and the plane mirror
surface is parallel to the transflective surfaces.
This arrangement provides a 1 to 2.sup.n optical distributor using
one mirror and only n beam splitter elements.
Preferably the splitter stages are arranged in order of beam
splitter effective length, with the shortest stage closest to the
optical input. In such a device the effective length of the beam
splitter is determined by the physical length of the transflective
surface.
According to a second aspect of the present invention there is
provided an optical distributor having 2.sup.n optical inputs
according to the first aspect of the invention, each of the
transflective means being in the optical paths originating from at
least two distinct inputs.
A first arrangement of such an optical distributer may comprise n
beam splitter stages in which each kth beam splitter stage has
2.sup.n-k spatially separate transflective surfaces arranged
between a first plane mirror surface and a second opposing plane
mirror surface parallel to the first plane mirror surface.
In this case each of the 2.sup.(n-k) transflective surfaces of the
kth beam splitter stage form 2.sup.k of transflective means of that
beam splitter stage. The stages are not necessarily in order as
will be discussed later.
In another arrangements the transflective means from different
stages can be formed from a single transflective element.
In such arrangements the transflective surfaces of the beam
splitter stages each act as beam splitters for beam originating
from two or more inputs.
A further redundancy in transflective means can be obtained in a
reflective star coupler according to the present invention as each
transflective means can form the transflective means necessary for
the optical path of at least four inputs.
Alternatively the transflective means of all the necessary beam
splitter stages can comprise portions of a single transflective
surface in which case the plane mirror surfaces include separate
mirror surfaces of different lengths arranged on either side of the
single transflective surface.
Preferably the beam splitters extend in the direction at right
angles to a first plane containing the optical input and outputs
and one or more further independent sets of optical inputs and
outputs are provided in a plane or planes parallel to the first
plane.
A distributor in accordance with present invention has the
properties of a star-network in the case with 2.sup.n inputs and
2.sup.n outputs or a tree-network in the simpler case of a single
input going to 2.sup.n outputs. It is readily constructed because
all the beam splitters may be identical in form, differing only in
scale for the different stages. This enables the use of a minimum
number of components to achieve the desired power distribution.
Since there is only one path between each input and output port,
multipath propagation and the resultant degradation in signal
handling cannot occur. The ratio of the device endface size to its
length is 2.sup.n /(2.sup.n -1) and tends to unity for large n. The
power distributor is therefore well proportioned and compact,
facilitating its insertion in line in optical systems.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
A device in accordance with the present invention is now described
in detail with reference to the accompanying drawings, in
which:
FIG. 1 is a side elevation of a first embodiment of the present
invention having a single input;
FIG. 2 is a side elevation of an alternative embodiment of the
present invention having a single input;
FIG. 3 is a side elevation of a further alternative embodiment of
the present invention in the form of a star coupler;
FIGS. 4 and 5 are side elevations of a star coupler functionally
equivalent of FIG. 3 with the beams splitting stages in different
orders;
FIG. 6 is a diagram illustrating growth-rules for the beam splitter
networks shown in FIGS. 1, 2 and 3;
FIG. 7a to 7c are diagrams illustrating the multiplexing functions
of devices in accordance with the present invention;
FIGS. 8a and 8b are side elevations of reflective networks in
accordance with the present invention;
FIG. 9 is a side elevation of a four by four coupler according to
the present invention in which transflective elements of different
stages are formed on a common element;
FIG. 10 is a diagram illustrating a 4 input common transflector
network in accordance with the present invention;
FIG. 11 is a diagram illustrating an 8 input common transflector
network in accordance with the present invention;
FIG. 12 is a diagrammatic perspective view showing the transmission
properties of a device in accordance with the present
invention;
FIG. 13 is a perspective view of a further embodiment of the
present invention; and
FIG. 14 is a perspective view of an alternative embodiment of a
switch incorporating a device in accordance with the present
invention.
FIG. 15 is a perspective view of an alternative embodiment of a
switch including a distributor.
DETAILED DESCRIPTION OF THE DRAWING FIGURES
Referring to FIG. 1 a device for distributing an optical signal
among eight outputs comprises an optical input 1 and three splitter
stages 2, 4, 6. Each splitter stage includes a transflective
surface lying at 45.degree. to the direction of incidence of light
from the input 1. A mirror 6 extends across the device parallel to
the transflective surfaces of the splitter stages 2, 4, 6. The
transflective surface of the second splitter stage 4 is twice the
length of the surface of the first splitter stage 2 and the surface
of the final splitter stage 6 is in turn twice the length of the
surface of the second splitter stage 4.
In use, a single beam is input at the optical input 1 which follows
the optical paths shown by the dotted lines to the outputs 01 to
08. The beam strikes the first beam splitter 2 and is divided into
two beams of equal power by a portion of the 50:50 transflective
surface of the beam splitter (constituting a transflective means).
One of the two resultant beams is transmitted through the beam
splitter 2 and passes directly to the second beam splitter 4. The
other beam is reflected from the transflective surface at
90.degree. to the one beam and is reflected through a further
90.degree. at the mirror sides so that after reflection the other
beam propagates parallel to the one beam but separated from it in
space. If losses at the mirror are neglected then since each beam
has undergone a single transition at a 50:50 transflective surface
each of the two beams reaching the second splitter stage 4 has half
of the power of the original input beam. The beams are similarly
subdivided at the subsequent splitter stages 4, 6 giving a total of
eight beams at the optical outputs 01-08. Each of the paths from
the input 1 to the outputs 01-08 involves three transitions at
50:50 transflective surfaces. The power of the input beam is
therefore equally divided with one eight going to each of the
outputs 01-08.
The transflective means of each stage form a single transflective
element corresponding to that stage.
The device may conveniently be constructed as a "sandwich" of
optical glass substrates. Transflective coatings are applied to the
substrates using masks to give the required coating geometry. The
coating used is chosen to have transmission and reflection
coefficients which are equal for light of different polarisation
states incident at 45.degree. and for a wide range of wavelengths.
The device is therefore polarisation and wavelength insensitive.
The substrates are then aligned by the outer edges and bonded with
optical epoxy resin. A mirror coating is then applied or a separate
mirror attached. Finally the "sandwich" is cut and polished to give
optical quality end faces.
An alternative construction shown in FIG. 2 uses an ensemble of
beam splitter cubes 12 of graded sizes packed with 45.degree.
prisms 14. This construction requires more sub-assemblies but is
physically suited to the fabrication of simple tree-networks with
only one input. In place of beam splitter cubes polarising beam
splitters followed by quarter wave plates may be used if the input
and output states of polarisation of the light are known to be
fixed.
In constructing the device the transflective coatings may be made
colour selective to give a wavelength responsive routing function.
In this case one or more of the beam splitting surfaces is arranged
to function as a dichroic mirror. At a given wavelength
substantially all of the light incident on the dichroic mirror is
reflected whilst light of a significantly differing wavelength is
transmitted. With a distributor including such wavelength selective
surfaces a signal may be wavelength encoded to determine at which
output or outputs it leaves the device.
The beam splitters may be made active to allow the interconnect
pattern to be rearranged dynamically. The splitters may, for
example, comprise polarising beam splitters and associated
electro-optic polarisers such as liquid crystal cells arranged to
function as half-wave plates. The input to the distributor is then
linearly polarised in a given direction and the optical axes of the
polarisers and beam splitters arranged at such an angle to the
direction of polarisation of the light that an incident beam is
transmitted or reflected according to the state of the
electro-optic polariser. Alternatively, active 1/4-wave plates
maybe used to produce circularly polarised components from the
incident plane polarised beam.
Referring now to FIG. 3, a device according to the present
invention configured as a star network is arranged to connect eight
optical inputs I1 to I8 with eight optical outputs O1 to O8. As
before there are, by way of example only, three splitter stages.
However in this example the first splitter stage includes four
separate beam splitters 16 the second splitter stage two beam
splitters 18 and the final splitter stage a single beam splitter
20. In general for a star network with 2.sup.n outputs and 2.sup.n
inputs then if the ensemble of splitter stages are considered in
order of increasing splitter lengths the kth splitter stage has
with this specific arrangement 2.sup.n-k separate beam splitters.
It should be noted that the splitter stages do not need to be
physically ordered in order of splitter length but may, for
example, be arranged in configurations such as those of FIGS. 4 and
5.
In use each input of the star-network effectively sees a tree
network which functions in the same manner as that described above
according to the first aspect of the invention. For clarity paths
from just one of the inputs is shown in FIG. 3. A light beam from
input I4 strikes a lower mirror 22 at 45.degree. and is reflected
upwards to the uppermost of the beam splitters of the first stage
16. Here it is split in two by the 50:50 transflective surface, the
reflected beam passing directly to the upper beam splitter of the
second stage 18 and the transmitted beam being reflected at an
upper mirror 24 before travelling parallel to the reflected beam to
the same upper beam splitter of the second splitter stage 18. A
beam can be traced in similar fashion each of the other input and
can be seen to be split in a manner precisely analogous to that
described above for the tree-network. As with the tree network each
path between the inputs 1I-I8 and the outputs O1-O8 undergoes the
same number of transitions at 1:1 transflective means and so the
power from any one of the inputs I1-I8 is divided equally between
the eight outputs O1-O8. The star-network is bi-directional: light
may be input from either end.
The tree networks for different inputs I1 to I8 have
transreflective portions in common because the two parallel mirror
22 and 24 provide that optical paths from different inputs approach
them from two orthogonal directions.
By increasing the number of beam splitter stages a device in
accordance with the present invention can be made for distributing
power from one or many inputs independently between a selected
number of 2.sup.n outputs. Growth rules for both tree and multiple
input star networks of the arrangements shown in FIGS. 1, 2 and 3
are shown in FIG. 6.
The propagation delay through the star network of FIG. 3 is a
linear function of input position with light from input I8
following the shortest path and hence emerging first and light from
input I1 following the longest path and hence emerging last. This
property may be exploited to give spatially-parallel to time-serial
or time-serial to spatially-parallel multiplexing operations.
Consider the 8.times.8 star couplers shown in FIGS. 7a to 7c. In 7a
a simultaneous but spatially split input to the device is output as
a signal which is spatially identical i.e. the same from each of
the spatially separated ports, but time-serial i.e. the output from
the lowermost port precedes that from the port immediately above it
and so on signal A being the first to exit from each. As shown in
FIG. 7b the device may be used to carry out the inverse operation
in an analogous manner by inputting at input I8 a time serial
signal (in which A enters the distributor first) and simultaneously
sampling the outputs as signal A exits output O8.
Either of the end faces of the device may be coated with a mirror
surface to give a distributor with the properties of a
reflective-star-network but with 2.sup.n multiple paths. However
since each multiple path has a propagation delay that is a linear
function of position this feature enables the use of the device as
a single-ended spatially-parallel to time-serial multiplexer as
shown in FIG. 7c. A plane wave pulse 26 is fed into the input port
I.sub.8 which is distributed to the eight output ports. The output
pulses impinge on a spatial modulator 28 serially, A which
therefore feeds back into the distributor time-serial inputs A to H
which then pass back to the input I.sub.8 to exit as a time-serial
signal 29.
Referring now to FIGS. 8a and 8b there are shown reflective
4.times.4 star couplers and an illustrative beam path for an input
at I.sub.4 which have transflective elements 28, mirrors 20 and 22
corresponding to those of FIG. 2 and additional mirrors 30 which
introduce additional redundancy in that the 4.times.4 coupler
requires only two transflective means each on each one
transflective element.
In the embodiments described with reference to FIGS. 1 to 5 the
transflective means which were formed by common transflective
elements were from the same beam splitter stage. This need not be
the case: in some configurations transflection means from different
stages can be combined, according to the present invention, on one
transflective surface to obtain a component reduction. This is
shown in the 4.times.4 star coupler of FIG. 9 in which stage one
and stage two means (I and II respectively) are combined on one
element. This enables a 4.times.4 coupler to be constructed from
only two transflective elements.
In alternative embodiments of the present invention shown in FIGS.
10 and 11, a single transflective surface forms the transflective
means for all the splitters stages. FIG. 10 is a 4.times.4 star
coupler utilising this configuration. Another such configuration is
shown in FIG. 11 is an 8.times.8 star coupler having a
transflective element 38, two common mirrors 40, 42 and three
double sided mirrors 44. In general for an N.times.N network where
N equals 2.sup.n, (N/2+1) mirrors are required including two
mirrors common to all stages. There are (log.sub.2 N)-1 stages, the
first stage having (N/4+2) mirrors, including the mirror common to
all stages. The second stage has (N/8+2) mirrors and so on, the jth
stage having (N/2.sup.(j+1) +2) mirrors. The internal mirrors,
which are double sided, are arranged about a point spaced mid-way
between the transflective surface and one of the common mirrors.
The common mirrors are placed on opposite sides of the
transflective surface and spaced from it by a distance proportional
to N/4. The common mirrors can if desired be extended laterally
from the positions shown in FIGS. 10 and 11 to intersect the upper
input and output beams so all the inputs and outputs are on the
same side of the device.
By increasing the number of beam splitters in the splitter stages a
device in accordance with the present invention can be made for
distributing power from one or many inputs independently between
many outputs. Growth rules for both tree and multiple input
star-networks of the form shown in FIGS. 1, 2 and 3 are shown in
FIG. 6.
By extending the transflective surfaces of the above embodiments in
the direction orthogonal to the plane of the inputs I1-I8 and
outputs O1-O8 the device may be depth-multiplexed with further
independent sets of inputs and outputs at intervals along the
X-direction as shown in FIG. 12, each independent set of inputs and
outputs lying parallel to the Y-Z plane. Such a depth-multiplexed
device may be used in combination with a un-multiplexed network of
the type described above in order to distribute a single input or
inputs over a two-dimensional array. Such an arrangement is as
shown schematically for a simple 4-output tree-network in FIG. 13.
A depth-multiplexed set of four tree-networks 61-64 is placed at
the output of a single tree-network 60. The depth-multiplexed
networks are rotated through 90.degree. with respect to the input
tree-network and arranged so that each output of the input
tree-network 60 goes to the input of a respective one of the
networks 61-64 of the depth-multiplexed set. Similarly a
depth-multiplexed set of star networks may be arranged to receive
the output of a single star network. Alternatively star networks
may be combined with tree networks in a similar fashion according
to the distribution characteristics required. Devices in accordance
with the present invention may advantageously be used in the
construction of broadband digital electro-optic space switches. The
low-loss and high optical quality of a device in accordance with
the present invention allows switch dimensions as high as
300.times.300 to be achieved. A further advantage of the use of
these devices in this context is that the switch throughput can be
increased with no loss penalty by using the two spatial dimensions
of the signal paths. For example, the signal may consist of a
32-bit data bus or even a complete page of data transmitted in
parallel. An architecture for a two-dimensional image switching
system is shown in FIG. 14. For simple 1-D serial transmission the
output optical elements may be replaced by a simple cylindrical
lens. Since inter-crosspoint signals are collimated optical beams,
the entire switch traffic can be routed in many ways. It is, for
example, possible to switch between M cross point arrays in order
to provide a fail-proof backup.
An alternative embodiment of a switch including a distributor in
accordance with the present invention, is shown in FIG. 15. The
outputs of a depth multiplexed set of N tree networks TI to TN are
input to N digitally indexed light deflectors DI to DN spatially
multiplexed in the direction at right angles to the depth
multiplexed tree-networks. The light deflectors comprise
birefingent crystals interspersed with electro-optic polarisers:
such deflectors are disclosed in the paper by Kulcke et al on pages
64-67 of the IBM Journal, January 1964. In the optical switch of
FIG. 14 the deflectors are used in an N input to 1 output
configuration so that the switch as a whole has N outputs extending
in a direction at right angles to the N inputs. Such a switch has
an inherent loss of 10 log.sub.10 N dB.
While the invention has been described in connection with what is
presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be
limited to the disclosed embodiment, but on the contrary, is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
* * * * *